The present invention relates to an ultrasonic motor and a laser beam welding apparatus and, more particularly, to a hollow ultrasonic motor which is suitably used in a laser beam welding apparatus for carrying out laser beam welding in the thin heat transfer tubes of a heat exchanger.
For example, a steam generator used in a nuclear power plant is a shell-and-tube type heat exchanger, in which some of thin heat transfer tubes may experience damage for various causes. As a convenient and reliable method for repairing those damaged tubes, it is well known to insert and fix a thin cylindrical member called a sleeve in the damaged portion of the heat transfer tube.
The method of fixing the sleeve in the heat transfer tube has been exemplified by soldering or expanding methods, but it has been found that laser beam welding is very effectual. We have developed a sleeve welding apparatus, as disclosed in the specification U.S. Pat. No. 4,839,495 (corresponding to Japanese Patent Laid-Open No. 27788/1989). This apparatus has, as described therein, an optical system including a reflecting mirror for directing a welding laser beam to the inner surface of the heat transfer tube or sleeve. In order to bond the sleeve and the heat transfer tube firmly by circumferential seal welding, it is necessary to turn the optical system about its axis, and as a rotation mechanism, therefore, an electromagnetic motor or a piston-cylinder mechanism is used.
For high-quality circumferential seal welding by using a laser beam, the rotational drive mechanism is required to produce a stable turning force with high torque and to rotate at a speed suited for the welding operation. In order to meet these requirements, generally speaking, the electromagnetic motor has to be equipped with a reduction gear mechanism, as disclosed in the specification of the U.S. patent, and thus it requires a relatively large mounting space.
In laser beam welding of this kind, on the other hand, the apparatus has to be equipped with optical fiber for transmitting the laser beam and hoses for feeding cooling gas or shielding gas. Moreover, the diameter of the heat transfer tube of the steam generator, into which the laser beam welding apparatus of this kind is to be inserted, is as small as 20 mm or the like, and the rotary drive mechanism is required to be smaller in size.
Incidentally, a heat transfer tube having a diameter exceeding 20 mm could be welded by a welding apparatus using an electromagnetic motor as a rotary drive mechanism. However, a laser beam welding apparatus for a heat transfer tube having a diameter less than 20 mm cannot be practicable as long as an electromagnetic motor is used.
In a rotary drive mechanism using a piston-cylinder mechanism, on the other hand, the linear movements of the piston in the axial direction can be converted into the turning motions of the cylinder having helical grooves. The control of the rotation is very difficult, although the aforementioned hose mounting space could be retained in the piston.